Bizarre repetitive behaviour may be quite common, with estimates in people and dogs of a prevalence of up to 8%. Whereas these are often considered important indicators of mental health problems in people, in companion animals, many owners view these behaviours as "quirky" or even amusing features of their pet. However, the absence of self-report also means that the array of repetitive behaviour conditions in companion animals is also much more diverse than in humans and there is a danger of drawing simple analogies between human and non-human animals based on very superficial phenotypic features. For example, the following differentials might be considered for a dog who chases its tail:
•Seizure possibly a primary partial seizure or secondary to a lesion in the brain such as damage to the vestibular system
•Inner ear infection
•Play
•Pain in the tail or lower back due to damage to the nerves (neuralgia), bones or soft tissues
•Pain around the region of the tail including skin inflammation and infections, impacted or infected anal sacs or trauma
•A learned response aimed at attracting owner attention.
•Acute conflict
•Compulsive Disorder
•Stereotypy

These explanations are not independent and a dog who starts chasing his tail for one reason may subsequently learn to do it at other times in order to gain attention, or the frequent repetition of the behaviour may potentially result in neurological sensitization and the expression of the behaviour becoming emancipated from its original triggering stimuli. From the simple (and by no means definitive) list of differentials for tail chasing given above it should be obvious that underlying cognition can vary enormously with a given presenting stereotypic behaviour. This presentation considers the value but also limitations of using companion animals as models of human conditions and especially the value of using real world rather than laboratory models.